1. Field of the Invention
The invention relates to a process for discharging a molten fluid from a furnace smelter into a dissolving tank. It is particularly applicable to chemical recovery furnaces such as those in which chemicals are recovered from the black liquor of wood pulp manufacture.
2. Description of the Prior Art
In the production of wood pulp, a chemical solution produced during one stage of the process is called black liquor and is obtained from wood being digested by the action of a chemical such as a mixture of sodium hydroxide and sodium sulfide. After the action of the chemical on the wood has been completed in a digester the residual liquor, usually called black liquor, contains salts which should be recovered from the standpoint of economical operation. The black liquor is evaporated to concentrate it and the concentrated black liquor is sprayed into a chemical recovery furnace which typically will comprise an oxidizing zone in an upper portion and a reducing zone in a lower portion. Most of the water remaining in the black liquor is driven off by the heat and drying is completed in the upper oxidation zone of the furnace. Dry solid particles are formed substantially free of moisture which collect on the bottom or hearth of the furnace. The combustible constituents of the dry particles are burned out and the heat that is generated is used for maintaining the chemical reactions taking place and also for producing steam in an associated boiler.
The inorganic ash remaining after the burning of the combustibles is fused by the heat of combustion. As this ash is melted, the oxidized forms of sulfur such as sodium sulfate, in the presence of carbon and a reducing atmosphere, are reduced to sodium sulfide. This sulfide together with other molten inorganic salts such as sodium carbonate is then removed from the furnace by discharge through a spout into a dissolving tank to form a solution known as green liquor. Discharge of the molten smelt into the liquid within a dissolving tank is generally accompanied by noise in the nature of a continuous roar at a high sound level and occasionally by violent and destructive explosions. Various systems have been proposed to eliminate or reduce this problem but none have been altogether successful and cost effective.
It has been found that the violence of the explosive reactions in the tank can be controlled by breaking up the stream of smelt issuing from the spout before the smelt comes into contact with the pool of green liquor in the tank. Typically, the shattering is accomplished by directing a jet of a gaseous medium such as steam against the stream of smelt which is leaving the spout. Since the quantity of smelt flow at any given time is highly variable, it is customary to direct an excessive amount of steam continuously against the smelt discharge at a pressure and velocity which would be adequate for breaking up even the heaviest smelt flow that would reasonably be expected. This technique obviously results in a considerable waste of steam and substantially reduces the economy of the chemical recovery process.
It has been proposed in Canadian Pat. No. 567,081 to direct a stream of steam vertically downward upon the flowing smelt and concurrently at a lower elevation within the tank to use a recirculated liquor stream to further shatter and disperse the smelt stream. The disadvantage of this process is that it relies upon continuous streams of both steam and green liquor.
U.S. Pat. No. 3,122,421 also describes an apparatus and method for dispersing or shattering a stream of smelt using steam. The novel aspect of this invention appears to reside in using the spout cooling water temperature as an indication of the amount of smelt flowing and therefore of the amount of steam required to shatter the smelt. Thus this patent also requires a continuous flow of steam although it may not be quite as wasteful as some of the other approaches. It also requires a water-cooled spout.
U.S. Pat. No. 4,011,047 describes a smelt spout for a recovery boiler which does not require water cooling. The spout is constructed from insulating and refractory material contained in a metal trough and is provided with a steam jet immediately adjacent the bottom free end of the spout. The jet impairs the formation of slag on the bottom of the trough and disintegrates the smelt stream issuing from the spout. Thus, this approach also requires a continuous flow of steam with its attendant economic penalty.